Bacteria are prokaryotic cells and do not contain any true organelles. On their surface, many bacteria have hairlike protrusions called pili that help them pass genetic information to other bacteria. Other bacteria are covered with a gelatinous capsule, which helps these cells to attach within tissues they will infect. The cell wall provides rigidity and support for the bacterium, while the cell membrane serves as a semipermeable barrier. Bacteria may also have external flagella that help the cell to move. Unlike their eukaryotic counterparts, which contain their DNA in a nucleus, bacteria have DNA that is coiled up inside a region called the nucleoid. The DNA is typically a double-stranded circular molecule. In addition bacteria may also contain small, circular extra- chromosomal DNA called plasmids. Bacterial cells reproduce by a process called binary fission. A bacterium starts with just one copy of circular DNA. This DNA is copied and each molecule of DNA is attached to the cell membrane. The cell enlarges, the membrane between the attachment sites grows inward, and a new section of cell wall is made. Two new daughter cells form from the one parent cell and the DNA returns to the nucleoid regions. This is an example of asexual cell division since one parent cell gives rise to identical daughter cells. Bacteria are tiny organisms that can be found on many surfaces, even on a chicken salad sandwich. Bacteria can divide quite rapidly, each cell dividing into two every 20 minutes or so. If there are two bacteria present at noon, after two hours there will be 128 bacteria. After four hours there will be 8200 bacteria. After six hours there will be 525,000 bacteria. After eight hours there will be 34 million bacteria in that small portion of the chicken salad.